The present invention relates to an adaptive line equalizer for correcting the distortions of digital siignals transmitted on a telephone subscriber loop.
Switched capacitor technology is known to implement an adaptive equalizer to meet the characteristics of transmission mediums of various line lengths. The gain of the equalizer at the Nyquist frequency (which is one-half of the maximum bit rate) is reduced when the peak value of the output of the equalizer exceeds a reference voltage and is raised when it drops below the reference voltage.
Since the equalizer is individually designed to equalize the transmission characteristics of a particular line length at the Nyquist frequency, there is a fixed relationship between the line length and the equalization characteristics. However, a drift in the transmission gain of the equalizer or a temperature-dependent line loss variation cause a long-term level variation to occur and destroy such relationship. Furthermore, in telephone subscriber loop applications, the bridged-tap connections are potential sources of echoes which necessitate the provision of a decision feedback restoration technique to eliminate intersymbol interference caused by the echoes.